1. Technical Field
The present invention relates to a receiving device that reproduces a digital broadcast, and particularly to a receiving device that reproduces a television broadcast program, a control method, a program and an integrated circuit for such a device.
2. Background Art
In recent years, the miniaturization and multi-functionalization of mobile terminal devices (for example cellular telephones and personal digital assistants) has progressed and models that can receive and display a television broadcast have been arriving on the market. For example, in Patent Document 1, an AV system for digital television broadcast reception is disclosed which displays the present time, the starting time and the ending time of a television broadcast program superimposed on a television broadcast display, immediately after changing channels. With this super-imposed display, it is possible for the user to easily ascertain the progress a television broadcast program is making.    Patent Document 1: Japanese Patent Laid-Open No. 2001-157131 Publication
However, for conventional mobile terminal devices that receive digital television broadcasts, it may take up to five seconds after the user changes channels until the television broadcast program on the chosen channel is actually displayed; in this interval, there is a possibility that the user may misperceive the device to be malfunctioning.
The problem is presented using FIG. 7. FIG. 7 is a schematic diagram that shows the picture types included in a digital broadcast according to MPEG-4 AVC standards. In the figure, it is shown that CHA is the original channel and CHB is the destination channel. “I” indicates an I picture which is intra-picture prediction-coded (intra predictive picture). “P” indicates a P picture (inter-predictive picture) which is inter-picture prediction-coded, using one reference picture. “B” indicates a B picture (inter bi-predictive picture) which is inter-picture prediction-coded, using two reference pictures. “IDR” indicates an IDR picture (instantaneous decoding refresh picture), a type of I picture, which indicates that all of the reference pictures stored in the reference picture memory in the decoder, may be invalidated. This IDR picture is a picture that can be decoded on its own without any reference pictures.
In the figure, time to is the time at which the user has changed channels from CHA to CHB. Time to is a time at which the first IDR picture included in CHB's digital television broadcast appears after the time t0. The mobile terminal device initiates decoding from the IDR picture of the time to. The interval between IDR pictures in a digital television broadcast is five seconds at most. Because of this, there may be cases where after the user changes the channel from CHA to CHB, it takes up to five seconds at most until CHB's television broadcast program is actually displayed. In contrast, for analog television broadcasting, from the user's point of view, the channels can be changed instantaneously. For users familiar with analog television broadcasting, there is a possibility that they may misperceive that the device is malfunctioning, since they cannot predict a digital television broadcast's waiting time.